In the future, new designer alloys for aerospace applications could be manufactured via the 3D laser melting process, thanks to the work of a researcher at the Swiss Federal Laboratories for Materials Science and Technology (Empa).
Titanium-Aluminium alloys are of great interest and use to aerospace engineers, due to their combination of low density, high strength, and their ability to resist oxidation at elevated temperatures. Christoph Kenel, in his PhD thesis, developed a novel titanium aluminide (TiAl) alloy intended for use in beam-based additive manufacturing technologies. This alloy includes nano-sized oxide dispersoids that improve their high temperature mechanical properties.
TiAl alloys are inherently brittle at room temperature, and the rapid solidification that additive manufacturing subjects it to can lead to complex phase transformation sequences, causing the alloy to fail and crack.
On the other hand, oxidation dispersion strengthened (ODS) alloys are a class of materials that offer deformation-, creep-, coarsening-, oxidation- and corrosion-resistance at temperatures up to 1,000 °C.
However, until now, it has been difficult to manufacture components using ODS alloys, due to cost and technical barriers. ODS alloys could only be created using classical powder metallurgy, in which oxides were added to alloy powders via ball milling in a solid-state process. Melting those composite powders would cause the loss of the oxide dispersoids, which would coarsen, dissolve and agglomerate on the surface of the molten metal as slag.
To deal with this challenge, Dr Kenel decided to develop a TiAl alloy specifically for the additive manufacturing process. He hypothesised that laser-based additive manufacturing can be used to create components from composite powders that contained oxide dispersoid, because the very short melting time and rapid solidification, hallmarks of additive manufacturing, could keep the oxide dispersoids well-dispersed within the alloy grains.
To develop the alloy, Dr Kenel used computational methods to simulate the phase transformations of the alloys during the heating and cooling conditions that additive manufacturing subject them to.
He also developed new experiments, including in-situ synchrotron X-ray micro diffraction methods during laser heating, that allowed him to systematically study the phase and microstructure formation in selected alloys under well-defined and simulated additive manufacturing conditions.
[These small-sized samples are made of oxide dispersion-strengthened titanium aluminides and have been printed as part of a PhD thesis. Photo: Empa]